The present invention relates to a method and an apparatus for continuously producing resinoid wheels for cutting hard metal materials.
Conventionally, resinoid bonded grinding wheels have been produced by placing a kneaded resinoid abrasive composition into a die of a given shape, smoothing the surface of the composition with raking means to give a uniform thickness to the mass of the composition, molding the composition at an elevated pressure and baking the molded product. However, this method has the drawback that relatively coarse abrasive grains are caught by the raking means and brought to the surface, rendering the resulting product uneven in grain size distribution. It has therefore been necessary to prepare abrasive grains of a uniform size by using a three-stage screening apparatus, but the procedure involved is very troublesome and takes a long time. Depending on the screens employed, moreover, about 10% of oversize grains and fines are separated off and discarded as being unsuitable for use. Thus the conventional grinding wheels are expensive to make. It is also noted that even with the use of the three-stage screening apparatus, the selected abrasive grains will have varying sizes, this inevitably entailing the foregoing drawback. Further according to the conventional method, the raked mass of the starting abrasive composition is pressed on one side for molding, with the result that there arises a difference between the pressures acting on the upper and lower surfaces of the molding. When the molding is baked which has been pressed on one side and is uneven in grain size distribution, the grinding wheel obtained becomes uneven in hardness, inasmuch as the product has high hardness where many coarse abrasive grains are present but low hardness where smaller grains are predominant. When put to use, the grinding wheel wears away more markedly where it contains many fine abrasive grains than where coarse grains predominate, so that an uneven wear takes place. As a result, the grinding wheel not only fails to cut a work straight but is also subjected to an objectional force and possibly broken in an extreme case. Moreover, if the abrasive composition is not fully raked, the resulting product will have a nonuniform thickness, consequently producing errors when cutting a hard metal material, and a markedly irregular portion of the grinding wheel, if any, will cause an objectionable force to act on and break the grinding wheel during use. In addition to the many drawbacks of the conventional method described above, the aforementioned raking means must operate with very high accuracy and is therefore expensive and requires a cumbersome maintenance procedure. The raking procedure is inconvenient in that it must be followed in a constant temperature and constant humidity chamber so as to render the abrasive composition free of changes in volume.